Backed by the U.S.-led international coalition, Iraqi forces fought their way Thursday into a sprawling military base outside of Mosul and onto the grounds of the city's airport, taking control of the runway amid fierce exchanges of fire with Islamic State militants.

The two-pronged advance is part of a major assault that started earlier this week to drive the Islamic State group from the western half of Mosul, Iraq's second-largest city

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The Iraqi federal police units, backed by regular army forces, entered the airport on Thursday morning, according to two police officials who said heavy clashes were underway hours later with IS militants hunkered down inside several airport buildings.

The officials said troops from the U.S.-led coalition were with the advancing forces, though they didn't specify the nationalities of the foreign forces. They spoke to The Associated Press on condition of anonymity because they were not authorized to talk to reporters.

Private broadcaster Lebanon-based Al-Mayadeen aired live footage from the Mosul airport perimeter, showing a military helicopter buzzing overheard and firing at IS positions as gunfire rattled.

Separately, Iraqi special forces entered the Ghazlani military base next to the airport on the southern edge of the city, the spokesman of the Joint Military Operation Command, Brig. Gen. Yahya Rasool told the AP.

Rasool said heavy clashes were underway inside the base. He did not provide more details.

On Sunday, after weeks of preparations, Iraqi forces officially launched the operation to take Mosul's western half, with the Iraqi regular army and federal police forces taking part in the initial push. Since then, the military says they have retaken some 120 square kilometers — nearly 50 square miles — south of the city.

Thursday marked the first time the Iraqi special forces, which played a key role in securing the eastern half of the city, joined the fight for western Mosul.

"The counterterrorism forces will be an additional force, which will expedite the liberation of Mosul's western side," Rasool told the AP.

In January, Iraqi authorities declared the eastern half of Mosul "fully liberated" from IS. The battle for western Mosul, the extremist group's last major urban bastion in Iraq, is expected to be most daunting yet.

However, the streets are older and narrower in the western section of the city, stretching west from the Tigris River that divides Mosul into the eastern and western half. The dense urban environment will likely force Iraqi soldiers to leave the relative safety of their armored vehicles. The presence of up to 750,000 civilians will also pose a challenge.

Mosul fell to IS in the summer of 2014, along with large swaths of northern and western Iraq. But the Sunni militant group has since consistently lost territory as the U.S.-led coalition proved to be critical for Iraqi government efforts to claw back territory lost to the extremists.

Also part of the Mosul battle but on its fringes, are government-sanctioned paramilitary forces, made up mainly of Shiite militiamen.

The militias this week launched their own new push to capture IS-held villages west of Mosul. The efforts by the militias, an umbrella group known as the Popular Mobilization Forces, are likely coordinated with government efforts.

The capture of Mosul International Airport and the adjacent Ghazlani base would be a major boost for the Iraqi forces in the fight for western Mosul. The Shiite militias already hold a small airport outside Tal Afar, a city located west of Mosul toward the Syrian border.